To provide a greater understanding of critical parameters of ultra-tight geologic formations such as shales, the current experimental methodology on whole-core samples is in need of improvement. Core-scale measurements are considered the “ground truth” that interested parties, such as oil and gas companies and electric utility companies, use to predict migration of deep underground fluids (for example, oil, natural gas, CO2 or brine) during extraction or injection operations. While prior art methods exists to make these assessments, interested parties often need to sacrifice measurement fidelity for a reasonable turnaround time, or vice versa. The present disclosure provides methods and apparatus to address this dilemma.
In recent years the industry has seen a dramatic shift in focus towards “tight” geologic formations such as shales, most notably as unconventional reservoirs for oil and gas exploration and as confining zones for carbon-sequestration operations. These formations have ultra-low permeabilities, meaning they are very difficult to penetrate. An accurate understanding a formation's permeability field is necessary to determine many operational parameters, such as but not limited to, ideal fracture spacing when drilling for oil and gas or to get accurate carbon-storage estimates. For example, a primary concern to the risks inherent in all carbon-sequestration operations is the prevention of upward migration of injected CO2 from the geologic formation in which it is stored through its overlying caprock formations to the atmosphere or into underground sources of drinking water (USDW's). The purpose of the overlying caprock formations is to provide long-term sealing of the sequestered CO2. Determining a safe amount of CO2 to inject into a given reservoir calls for a comprehensive understanding of the integrity of its caprock. Part of this understanding involves accurate estimation of the permeability and porosity of the confining zone.
Making a quantitative permeability assessment always begins with the “ground truth” of core-scale measurements, but using conventional techniques to analyze ultra-low-permeability materials is notoriously onerous and time-consuming [1]. To overcome this limitation, experts across multiple disciplines have developed and refined a technique called “pressure-pulse decay” [2, 3], culminating in an experimental methodology referred to as the Gas Research Institute (GRI) method [4]. The GRI method is fast, but cannot be employed under in-situ conditions; nor is it repeatable across multiple laboratories [5, 6].
As previous investigations of pressure-pulse-decay measurements of fractured samples have demonstrated, one very influential indicator of experimental duration is the amount of sample surface area exposed to the pulse [7]. The GRI method took this concept to its logical conclusion by crushing the sample into small cuttings and analyzing a pulse decay applied to them [4]. However, since the size, shape and orientation of each cutting is arbitrary, defining a representative geometric domain for any model to compare against the resultant data is not feasible. Thus, the accuracy, repeatability and representativeness of the GRI model cannot be ensured, as reported throughout the technical literature [8, 9].
The present disclosure provides both methods and apparatus to simultaneously determine the porosity and permeabilities parallel and perpendicular to the native bedding planes using a single test on a single sample whose geometry is easily defined, such as for example a cylindrical core sample. Such methods and devices were not previously appreciated in the art.